As the most stable form of carbon, graphite has found use in a variety of industrial applications. For instance, due to high resistance to erosion and thermal fatigue, graphite has been used for many years in formation of molds for casting of metal parts such as gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons, just to name a few. Graphite molds are commonly used for molding of metals and metal alloys including alloys of aluminum, magnesium, copper, tin, zinc, and lead as well as iron and steel, nickel, cobalt and/or iron based superalloys, stainless steel alloys, titanium alloys and titanium aluminide alloys.
Graphite has also been commonly used in foundry operations as a facing material and as a lubricant. For example, a useful foundry facing mold wash is a water-based paint of amorphous or fine flake graphite. By painting the inside of a mold with the wash and letting it dry, a fine graphite coat can remain on the surface of the mold that can ease separation of the cast structure following formation and cooling. Graphite lubricants are specialty items for use at very high or very low temperatures as a forging die lubricant, an anti-seize agent, and as a gear lubricant for machinery. High film strength graphite lubricants are also utilized during formation of metal wires to prevent metal to metal contact during wire drawing.
In many instances, it becomes necessary to remove graphite from associated materials. For instance, following casting of a metal piece, graphite of the mold or mold facing should be removed in a cleaning process. Similarly, formation processes that utilize graphite lubricants in the formation (e.g., wire drawing) should include removal of the graphite from the piece.
Traditionally, removal of graphite from another material has been carried out by immersion of the piece into a bath of molten salt at elevated temperatures (e.g., about 500° C.) for several hours. Unfortunately, an amount of the graphite often remains adhered to the surface following the bath and this must be removed by physical means such as blasting, chipping, or drilling the debris away by hand. Such removal methods are frequently dangerous and inefficient, particularly in those cases in which the piece has a complicated geometry, such as a cast piece including a hollow internal element formed with a mold core (a solid component of a mold that provides hollow internal elements within a cast metal part), a wire coil, and the like. Even with combination of a molten salt bath and physical removal processes, complete removal of graphite from a piece can often prove difficult.
Molten salt bath cleaning also proves problematic as complete immersion of the piece is necessary and this can prove particularly difficult when considering cleaning of large, irregularly shaped pieces. Moreover, the reactant salt will be consumed during the cleaning process and the carbonate reaction product will build up in the bath as the graphite reaction proceeds, leading to the necessity of refurbishment of the bath at regular intervals. For instance, only about 25% of a sodium nitrate bath can be efficiently utilized before the bath must be reformulated. Thus, large salt waste volumes are generated and the overall process can be very expensive.
What are needed in the art are methods for removing graphite from other materials. For instance, a processing method that can efficiently remove graphite casting materials and graphite lubricants from a formed metal piece and graphite matrix from nuclear fuel particles would be of great benefit.